1. Field of the Invention
This invention relates to apparatus for attenuating waves, and more particularly, to coaxial attenuators for attenuating baseband pulses with components in the microwave frequency range.
2. Prior Art
The use of coaxial attenuators for attenuating high frequency waves is well known in the art. In all the prior art coaxial attenuators resistive elements, in some form or other, are inserted between the inner conductor and the outer conductor and in series with the inner conductor. These elements attenuate the pulses as they are propagated along the coaxial line. There are several configurations of these prior art coaxial resistive attenuators, such for example, as the T-pad attenuator and the line type attenuator. Each type of the presently-used attenuators has basic limitations which will be presently explained and with which the present invention is concerned.
The T-pad attenuator consists of a series of fixed resistive elements of T-formation, which gives a definite attenuation with a constant input and output impedance, depending upon the resistance values selected. However, there is a condition that the individual resistive elements will stay constant in value only below the frequencies where the wave length is long compared to the physical dimension of the resistors. When this condition fails, the value of the resistors, and therefore attenuation of the unit, will change with frequency. As a result of this change, the T-pad attenuator cannot be used for attenuating pulses with components in the high microwave frequency range. In fact, the best T-pad attenuator has a cut-off frequency of approximately 10 GHZ. However, in several applications, for example, laboratory experiments, it is necessary for one to obtain the attenuation of pulses with components in the high microwave frequency range (up to approximately 100 GHZ) and due to the limitation discussed above, the T-pad attenuator is not satisfactory.
Another type of resistive attenuator is the so-called line type attenuator, also known as the distributive type attenuator. In this type of attenuator, the inner conductor of the coaxial line is replaced with a resistive element, so that the field of wave traveling down the coaxial line is attenuated because part of the energy is dissipated in the resistive elements. With the line type attenuator, waves can be attenuated to a higher frequency (20 GHZ) than the T-pad type attenuator, although not as high as one would disire. However, there is a low frequency limit due to the length of the resistive element compared with the wave length; in this case, the length of the resistive element should be long compared with the wave length. Due to the limitation of the line type attenuator in attenuating waves in the lower and the upper frequency ranges, the device is essentially a narrow band device and is useless for very wide band applications.
Still another type of resistive attenuators is the so-called card-type attenuator. Basically, the card-type attenuator consists of a flat insulating plate, usually of ceramic material, having a thin conductive or resistive coating on at least one surface thereof, which coating acts as the attenuating element. Although this attenuator attenuates wave to a lower frequency range than the line type attenuator, it has the same limitation as the T-pad type attenuator, i.e., inability to accurately attenuate very fast pulses. The failure to attenuate waves, uniformly with frequency, in the high microwave frequency range stems from the fact that the disk type attenuator has stray reactive elements, for example, capacitive components, which results in a change in attenuation with increased frequency (a result which can be regarded as principally due to the effect of the ceramic disk).
In addition to the inability of the prior art attenuators to attenuate waves in the high microwave frequency range, (i.e., up to and above 100 GHZ); the pulse waveforms which are attenuated by the prior art attenuators are distored, i.e., the pulse waveforms after attenuation are changed in shape. In fact, the best available coaxial attenuators have a band width from DC up to some 20 GHZ. The corresponding step response risetime is about 20 psec with typical .+-. 10% overshoot and ringing. For precise picosecond pulse measurements, these specifications are inadequate. This stems from the fact that the resistive elements of the prior art attenuators are not constant in frequency, i.e., as the frequency range is increased, the prior art attenuators are plagued by the effects of the stray elements (series inductance, shunt capacitance, etc.) which changes the effective value of the resistive element and as such, accurate attenuation is not achieved.